Motor vehicles emit hydrocarbons as a result of the evaporation of fuel. Generally, such evaporative emissions result from the venting of fuel vapors from the fuel tank due to diurnal changes in ambient pressure and/or temperature, the vaporization of fuel by a hot engine and/or exhaust system, and the escape of fuel vapors during refueling of the vehicle. The venting of fuel vapor from the fuel tank due to diurnal pressure and/or temperature changes (i.e., diurnal emissions) is responsible for a majority of evaporative emissions. Diurnal changes in pressure and/or temperature cause air to flow into and out of the fuel tank. Air flowing out of the fuel tank inevitably carries fuel vapor which is created by the evaporation of fuel into the air contained above the fuel within the fuel tank. If this flow of air is left untreated and is allowed to escape directly into the atmosphere, undesirable emissions occur.
One way in which motor vehicle manufacturers have reduced the level of diurnal emissions is through the use of evaporative canisters. A detailed discussion of the structure and operation of an evaporative canister is presented in U.S. Pat. No. 5,910,637, the disclosure of which is incorporated herein by reference.
Generally, an evaporative canister has a vapor inlet, a purge port and a vent port. The vapor inlet is fluidly connected by a vapor conduit to the air space in the fuel tank. Diurnal changes in pressure and/or temperature causes air within the fuel tank to flow through the vapor conduit and into the evaporative canister via the vapor inlet. The air carries fuel vapor and/or hydrocarbons. The evaporative canister contains a sorbent material, such as an activated carbon, that strips fuel vapor from the air as it flows through the canister. The treated air then flows out the vent port and into the in atmosphere. The purge port is fluidly connected by a valved purge conduit to the combustion air intake of the motor vehicle engine. When the engine is running, the combustion air intake is at sub-atmospheric pressure, and the valve is opened to thereby connect the purge port to the combustion air intake. Fresh air is drawn by the sub-atmospheric pressure through the vent port and into the evaporative canister. The fresh air flows through the sorbent material, out the purge port and into the combustion air intake. The flow of fresh air through the evaporative canister strips the sorbent material of stored fuel vapor and/or hydrocarbons, thereby purging the evaporative canister of hydrocarbons.
However, minute levels of hydrocarbons remain stored in the sorbent material of a purged evaporative canister. Bleed emissions are believed to result from the release of these stored hydrocarbons (i.e., the hydrocarbon heel) from the evaporative canister into the atmosphere. Bleed emissions typically occur, for example, during the heating of the fuel tank during a diurnal cycle. The heating of the fuel tank causes air to flow from the fuel tank, through the canister, out the vent port and into the atmosphere. The air carries the hydrocarbon heel out of the canister and into the atmosphere, thereby resulting in the release of bleed emissions.
In order to reduce bleed emissions some motor vehicles employ an auxiliary canister. The auxiliary canister is placed in series with and further filters the treated air flowing out the vent port of the main evaporative canister. The auxiliary canister typically uses the same sorbent material, i.e., granular or pelletized carbon, as is used in the main evaporative canister to thereby increase the hydrocarbon capacity of the evaporative emission control system. However, in order to achieve sufficient hydrocarbon capacity, auxiliary canisters are generally highly restrictive to the flow of air. Thus, the auxiliary canister must be bypassed in order to be compatible with vehicle refueling vapor recovery systems. Bypassing an auxiliary canister requires the addition of valving and conduits to the evaporative emissions control system, and thus adds cost and complexity to the system. Furthermore, the restrictive air flow characteristics of the auxiliary canister makes purging the volume of sorbent material inefficient, especially in small displacement engines. Moreover, vehicles which incorporate a more efficient evaporative canister and/or an auxiliary canister typically do not reduce bleed emissions to a level required to classify the vehicle as a Super Ultra Low Emissions Vehicle (SULEV) or as a Practically Zero Emissions Vehicle (PZEV).
Therefore, what is needed in the art is a device which reduces the bleed emissions of an evaporative canister and/or the combination of an evaporative canister and an auxiliary canister.
Furthermore, what is needed in the art is a device which reduces the bleed emissions from an evaporative canister and has a low flow restriction, thus rendering the device compatible with vehicle refueling vapor recovery systems.
Yet further, what is needed in the art is a device which reduces bleed emissions from an evaporative canister and has a low flow restriction, thereby increasing purge efficiency.
Even further, what is needed in the art is a device which reduces bleed emissions from an evaporative canister and which has a higher efficiency than an auxiliary canister utilizing carbon pellets or granules.